Manufacture of interior coated bulbs for high temperature glass lamps

ABSTRACT

AN INTERIOR COATED LAMP ENVELOPE HAVING A THICK-WALLED BULBOUS PORTION IS MANUFACTURED BY FIRST COATING AN INTERIOR REGION OF A LENGTH OF QUARTZ TUBING WITH A LAYER OF BORIC OXIDE. THIS MAY BE DONE BY BUBBLING NITROGEN GAS THROUGH LIQUID TRIMETHYL BORATE AND FEEDING THE RESULTANT VAPORS, ALONG WITH AIR, THROUGH THE QUARTZ TUBING WHILE ROTATING IN A GLASS LATHE AND HEATING THE REGION THEREOF WHICH IS TO BE FORMED INTO A THICK-WALLED BULBOUS PORTION. WHILE CONTINUING THE HEATING, THE ENDS OF THE TUBING ARE FORCED TOWARD EACH OTHER THEREBY FORMING A THICK-WALLED BULBOUS PORTION AND AT THE SAME TIME BAKING THE DEPOSITED COATING OF BORIC OXIDE.

M Q l2 25, 1972 s. P. SENFT I 9,

- MANUFACTURE OF INTERIOR COATED BULBS FOR HIGH TEMPERATURE GLASS LAMPSFiled Sept. 18. 1970 unm- ITWVEZTT't'OTI Stephen P Sewf' t b9 91W, 62111.

His A1."tZOT"TWG H United States Patent vs. c1. 65-60 a 6 ClaimsABSTRACT OF THE DISCLOSURE An interior coated lamp envelope having athick-walled bulbous portion is manufactured by first coating"aninterior region of a length of quartz tubing with a, layer of boricoxide. This may be done by bubbling nitrogen gas through liquidtrimethyl borate and feeding the resultant vapors, along with air,through the quartz tubing while rotating in a glass lathe and heatingthe region thereof which is to be formed into a thick-walled bulbousportion. While continuing the heating, the ends of the tubing are forcedtoward each other thereby forming a thick-walled bulbous portion and atthe same time baking the deposited coating of boric oxide.

BACKGROUND OF THE INVENTION The invention is in the field of lampmanufacture, and is directed to the manufacture of interior coated bulbsfor lamps which operate at high temperature.

US. Pat. No. 3,263,852 to Elmer G. Fridrich (assigned to the sameassignee as the present invention) describes a method of forming aquartz envelope having a thickwalled bulbous portion, in which method alength of quartz tubing is chucked at both ends in a glass lathe, and,while heating the quartz tubing in its central region, the chucks aremoved relatively toward one another. US. Pat. No. 3,305,289 to Elmer G.Fridrich (assigned to the same assignee as the present invention)describes a method of sealing electrodes in such a thick-walled bulbousenvelope, to provide an arc lamp.

The above-described arc lamp is intended to operate at a hightemperature, for example greater than 1000 C. and preferably at 1400" C.However, at such high temperatures the inner surface of the bulb tendsto devitrifyi.e., it crystallizes and develops small cracks, which causereduction of light output and also physical weakening of the bulb,thereby shortening the useful lamp life. US. Pat. No. 3,390,298 to HorstWerner (assigned to the same assignee as the present invention)describes a boric oxide treatment of the inner surface of the lamp bulb,which causes the inner bulb surface to become molten during lampoperation and prevents the occurrence of devitrification. The boricoxide may be applied to the bulb by filling it with a solution oftrimethyl borate, draining, and baking at 1200 to 1300 C. for two hours.Alternatively, the borate may be applied inside the bulb by a flow ofvapors of trimethyl borate entrained in an inert gas such as nitrogen,while heating to about 700 C. and supplying air to oxidize the trimethylborate and produce the boric oxide coating. Instead of using trimethylborate, tributyl borate or diborane may be used.

Although the above-described boric oxide treatment of the bulb iseffective in preventing devitrification of the bulb during lampoperation, it requires considerable time and increases the cost ofmanufacturing the lamps.

SUMMARY OF THE INVENTION Objects of the invention are to provide animproved method of manufacturing lamp bulbs, and to provide an improvedmethod of applying boric oxide coating inside bulbs for high-temperaturelamps, resulting in reduced manufacturing time and lower cost.

3,679,385 Patented July 25, 1972 The method of the invention comprises,briefly and in a preferred embodiment, the steps of coating an interiorregion of a length of tubing offused vitreous silica, such as quartz,with a borating material, heating said region of the tubing, and forcingthe ends of the tubing relatively toward each other, thereby forming athick-walled bulbous portion and baking the borating material to produceboric oxide.

BRIEF DESCRIPTION OF THE DRAWING 1 illustrates the step of introducing aborating material into a" region of quartz tubing which is chucked inaj' 'gla ss lathe, while at thesam'e time heating said region of tubing,and... 7' I v 2 illustrates the step of forcing the ends of thequartztubing relatively toward 'each other while heating the regionthereof to which the boratingmaterial has been applied.

DESCRIPTION OF Til-IE PREFERRED EMBODIMENT As shown in FIG. 1, a lengthof tubing 11 of fused vitreous silica, such as quartz, is held in androtated by a chuck 12 of a glass lathe. A flame burner 13 is positionedto heat a region 14 of the tubing 11 to a temperature of at least 700 C.A vessel 16 contains a solution of borating material, such as trimethylborate, tributyl borate, or diborane. A nozzle or tubing 18 extends intothe borating liquid 17, and an inert gas such as nitrogen is flowed intothe tubing 18 and bubbles through the borating liquid material 17. Anozzle 21 extends from the vessel 16 and partly into the quartz tubing11, so that the inert gas, carrying vapors of the borating material 17,enters the quartz tubing 11 at the region 14, whereby the boratingmaterial deposits on the inner surface of the tubing 11 as the heatedregion 14. An air inlet 22 is provided, for combining air with the vaporcarrying nitrogen, so that the borating material will oxidize wheredeposited at the heated region 14 inside the tubing 11 and form boricoxide.

The borating nozzle 21 is removed from the quartz tubing 11, and thefree end of the tubing 11 is chucked in a second chuck 23, as shown inFIG. 2., and, while continuing to heat the region 14 of the tubing bymeans of flame burner 13, the chucks 23 and 12 are forced relativelytoward each other, as indicated by the arrow 24, thereby forming athick-walled bulbous portion 26 in the manner described in theabove-referenced Pat. 3,263,852, while at the same time baking thedeposited borating material on the inner wall of the region 14. Thus, ina continuous operation, the boric oxide layer is formed within thebulbous portion 26, and baked, and the bulbous portion 26 is formed.This simple continuous operation saves considerable time inmanufacturing the coated bulb, and reduces the manufacturing cost,because it eliminates the steps, previously deemed necessary, of coolingthe formed bulb after it is made, storing it, and then chuck ing itagain in a glass lathe and heating up the bulbous portion whileintroducing the borating material.

Instead of removing the nozzle 21 before chucking the free end of thetubing '11 in the chuck 23, as described above, both ends of the quartztubing may initially be chucked as shown in FIG. 2, and a nozdle 21 maybe inserted through a hollow portion of one of the chucks 23 and into anend of the quartz tubing 11, in order to introduce the borating materialtherein. If desired, the borating material may thus be introduced intothe tubing 11 at the same time as the chucks 23 and 12 are being movedrelatively toward each other for forming the bulbous portion 26. In amodification of the method above-described, air may continue to beflowed through the intake 22 and out of the nozzle 21, after the flow ofnitrogen has been terminated in the tubing 18, so as to facilitateoxidizing the borate material while it is being bakedduring theformation of the bulb portion 26. During the latter stages ofthelafore-described continuous method, it is desirable to increase the;temperature of the heated region. Also, if desired, the borating vaporsmay be fed into the region 14 before, during, and/ or after formation ofthe' bulb portion 26, taking advantage of the bulbgf orrr'iing heat fordepositing and baking the layer of borating material within the bulbportion 26.

' In ajpr'eferred embodiment of the invention, the flow "rate of theinert gas in tubing 18 isapproximately 0.1 to 0.03 c.f.h., the flow ofair into tubing 22 is'at a rate of about:0.5=to 1.0 c.f.h., the timeduration of flowing the ,vaporsinto the bulb region 14 is approximately5 to 40 seconds, and the amount of boric oxide formed in a typical lampenvelope, having a bulbous volume of less than 1 cubic centimeter, isabout 0.1 to 6.0 milligrams. While a preferred embodimentpandmodification of the invention have been shown and described, variousother embodiments and, modifications'will become. apparent to personsskilled in the art, and'will fall within the scope of invention asdefined in the following claims.

What I claim asnew and desire to-secure by Letters Patent of theUnited'States is: v

1. A method of making a lamp envelope having a bulbous portioninteriorly coated with boric oxide, cornprising the steps of introducinga borating material into a' length of tubing of fused vitreous" silicaat a region where said bulbous portion is desired,heating said region,

and forcing the ends of the tubing relativelytoward each other therebyforming said bulbous portion at said heated region and baking saidborating material to form said coating of boric oxide, said steps beingperformed in'a continuous operation.

2. A method as claimed in claim 1, in which said borating material isintroduced into said tubing prior to said forming of the bulbousportion, whereby the borating material is baked during the entire timesaid regionof the tubing is heated for forming said bulbous portion.

3. A method as claimed in claim 1, in-which said borating material istrimethyl borate.

4. A method as claimed in claim 1, in which said borating material is.tributyl borate. w

5. A method as claimed in claim 1, in which said borating material isdiborane. I 6. A method as claimedin claim 1, in which air is flowed insaid'tubing during said heating of said region thereby'facilitating theformation of said boric oxide.

7 References Cited 6 UNITED STATES PATENTS 1,948,560

